Because of their unique combination of mechanical properties, thermoplastic polyurethanes (hereinafter TPU) have been extensively used in a variety of molding applications. Articles molded therefrom have met the stringent engineering criteria governing applications in the automotive, electrical, mechanical equipment and footwear industries. However, the flammability rating of TPU (UL-941/8", V-2) limits their utility in certain applications where a greater resistance to flame is required. Halogenated fire retardants, singly or in combination with antimony oxide have been reported to improve the flame retardance of these resins but are known to be unsuitable for some applications where corrosion upon evolution of hydrogen halides attended upon combustion is a determining factor.
Thermoplastic polyphosphonates are characteristically of exceptionally high rating of flame resistance--see, for instance, German DOS No. 29 44 093. Due in part to their level of mechanical properties, however, thermoplastic polyphosphonates are beneficially used in a narrower filed of application.
Polyblends of TPU and certain thermoplastic polyphosphonates purposed at combining their individually useful properties have been reported. It has been, however, the experience of those skilled in the art that certain thermoplastic polyphosphonates desirable as components in a blend with TPU are not compatible therewith and the resulting blends are inhomogeneous, poorly dispersed mixtures which are unsuited for molding applications.